Method of making corrosion-resistant steel



Ufli m Pa Charles WaWheatley, Milwaukee, Wis., assignor to A. O.

Smith Corporation, Milwaukee, Wis.,a corporation of NewYo'rk 3 No Drawing. Filed nears, 953, se se. 134,831

:1 Claim. CI. 75-56) This inyention relates to a-method of making steel having an increased resistance to corrosion from an alkaline'medium. It has been found that steel having a low silicon con tent and a plurality of oxygen rich areas throughout the structure exhibitsl excellent resistance to corrosion from an alkalinefmedium-anjd this resistance is particularly evident when; the rolledsteel contains banded layers or striationsbfoxides p 5 i t The present'inve'ntionis directed to amethod of making this specially processed steel in which the oxides are present in meronea product in the form of striations orbands'. 1 i 1 According to the invention the method consists of uniformly adding finely dividedmill scale or other relatively impure form s of iron oxide, such as iron ore, to Tthesteel during teemingin an amount equivalent to at least 1.75

2,983,598 Patented May 9, 1961 The carbon content, by ladle analysis, while maintained under 0.30%, should generally be between about 0.15% and 0.30% so that the resulting steel will have a carbon content of 0.12% to 0.27% to provide adequate mechanical properties to be utilized for structural purposes such as the fabrication of vessels or the like.

It is essential that the silicon content be maintained below 0.10% for it has been found that a silicon content above this maximum results in a considerable decrease in the corrosion resistance of the steel to alkaline attack. i

To obtain the desired oxygen rich areas and striated form in the rolled product, mill scale, or other unpurified forms of iron oxide, such as iron ore, is added into the ingot mold. The mill scale which is essentially Fe O or'a combination of Fe O =Fe O and FeO, is added during the teeming of the steel so as to produce the eventual distribution of the oxides in the steel necessary to achieve the desired resistance to alkaline corrosion of the plate steel or wrought products into which the ingot isultimately formed.

pounds of mill scale per ton of steel andthereafter plating l or capping the steel ingot to solidify the steel and terminate the reduction of theoxides by the carbon present in the steel. Terminating the reduction of the oxides results in the steelretaining acarb on content in excess of 0.12%andhaving substantially uniformly'distributed areas of oxides throughout' the "ingot; The ingot issubjsequently rolled into plate of wrought 1 products and the rolling deformsthe oxygen rich' a reas into striations or bands which oif eflinaximum.resistance' to corrosive attack from alkaline 'inedium:=

i In the past, mill scale has been added in small amounts tokeep the heat open and increase theamount of rim. Generally the mill =scalehasbeen added to the ladleb'ut occasionally mill scale, in smallamounts up to 0.5 pound per ton of steel, .has beeii' added-E0016 ingot 'inold to keep the heat "open: I-Iojwever", mill scale added to *the ladle or added to -=the -in'old in these small amounts"will not result in the oxygen richareas whichrare essential" to the present invention nor will it result in the oxides being The mill scale is added in a quantity of at least 1.75 pounds'per ton of steel with a range of 2.5 to 6.0 pounds of mill scale or its equivalentper ton of steel having been found to be very satisfactory for most purposes.

The mill scale is added to .themold in a finely divided state to achieve a uniform distribution of the oxides in the steel. It has been found that discrete particles having a size of ,4 inch or smaller will produce this desired uniform distribution. The use of larger sized particles or lumps will result in a less uniform distribution of the oxides in the steel and in extreme cases the mill scale may not go into solution in the steel but will remain as included areas of mill scale, which areobjectionable. The finely divided mill scale or equivalent form of iron oxide is added generally continuously and uniformly to the ingot mold during teeming at a rate determined by the operator an dthe effervescence of the metal to obtain a uniform distribution of the oxides in the metal. Generally, the operator adds the mill scale to the mold during the entire pouring operationat the maximum rate possible without causing the steel to boil up over the top present in ite desired striated form. By adding the mill scale or other equivalent form; ofwiren pxides in an amount of at least 1.75 pounds per ton duringteeming andthereafteiplatingor-capping theingot to prevent the "completereajetion.betweenthecarbon of the steel and .the'

iron oxides. can. thedesired oxygenrcontent .inistriated Carbon 0.30% maximum. Manganese 1.00% maximum. Silicon 0.10% Total phosphorus, sulphur and trace elements 1.00% maximum. Ir Balance.

of the mold. The addition of mill scale at this rate will result in the 2.5 to 6.0,pounds of. mill scale per ton of steel range setforthtabove. a 1* 5. Inorder to produce the oxygen rich areas in the steel as Well as maintaining the carbon contentabove 0.12%

so that the, steel will have adequate mechanical properties, it is essential that the mill scale be added to the ingot mold during teeming and that the mold be plated or capped toj solidify thesteel and minimize the reduction of the oxides by the carbon present in the steel. The addition ofmill'scale or the equivalent form ofiiron oxide to the ladle, instead of to the ingot mold, will not produce the desired pro'perties .in 'the'steel,.for a more complete reaction between the oxides and the, carbon of the, steel will ccur, resulting i both a reduced. oxide content and a decreased 5: on conltent in thesolidified steel. .1 "In the resent invention the min scale .is added to "the mold during the entire pouring of the steel into the mold, and on completion of pouring, the ingot is capped or plated to terminate the reduction of the oxides by the carbon. The-plating is done after a given thickness of solidified steel crustforms on the outer surface of the ingot. the thickness of steel crust is judged by the operator, and the required thickness before plating depends largely on the size of theingot and is usually about 4 to 6 inches. Generally, the time between the completion of pouring and the plating of the ingot is from 10 to 20 minutes. p

The plating or capping of the ingot serves-to cool and solidify the top layer of the steel and prevents the evolution of gas from the steel. The plating in effect terminates the reaction between the oxides and the carbon of the steel and prevents complete reduction of the oxides by the carbon. This results in the steel retaining a carbon content in the range of 0.12% to 0.27% and also having uniformly distributed oxygen rich areas which appear in the1 desired striated form when the ingot is subsequently rol ed. a

After the ingot hassolidified it is transferred to a soaking pit which is at a temperature of about 1800 F. to 2000 F. When the temperature of the ingot has reached equilibrium with the temperature of the soaking pit, the ingot is heated to the rolling temperature of about 2400 F. After the ingot has been heated through to the rolling temperature it is removed for rolling. Soaking or. prolonged periods of heating of more than one hour at the rolling temperature results in a diffusion of the oxides steel at this temperature for a period of leasttw o hours, the banded condition of the oxides can be further defined and accentuated.

The oxygen content of the plate may be determined by means of vacuum fusion in a graphite crucible. However, as the oxides in the plate are not uniformly distributed throughout the plate, but instead are concentrated as bands or striations, it is difficult to obtain an accurate determination of the oxygen content. Generally speaking, the oxygen content of the plate should be in the range of 0.005% to 0.05% with a preferredrange .Of 0.01% to 0.03%, but as previously set forth, thebanded .or

J striated form of the oxides brought about by the aforewhich is undesirable, and thus the ingot is removed from v the soaking pit shortly after being heated through.

v, The ingot is rolled by conventional practices normally used in the production of steel plate or wrought products, and the rolling operation tends to product striations or banded layers having a high concentration of oxide inclusions. These banded layers are believed to consist of the oxide inclusions surrounded byareas of iron supermentioned addition of mill scaleis'the feature which is most important in bringing out the improved corrosion resistance to the alkaline media rather than the specific oxygen content of the plate.

The present invention provides a simple and inexpensive method of producing a steel having a plurality of oxygen rich areas throughout the structure disposed in a banded form. The high oxide content and banded condition of the same, results in a steel which exhibits excellent resistance to corrosion from alkaline solutions, such asthat encountered in sulphate digesting processes.

Thi pplication is a continuation-in-part of application Serial No. 482,187, filed January 17, 1955, .now abandoned, and entitled Method of Making Corrosion Resistant Steel. I

pounds of mill scale to a 20 ton ingot during teeming. f

The sample was takenfrom a rolled plate representative of the center portion of the ingot. The ladle analysis of the steel and the; analysis of the sample are as follows in weight percent: 7 p

"M11 P i 0 Si :j Fe

-.5i .013. .030 .009 Balance. -56 -.012 .032 .004 Balance.

Definite bands'o'r striations, "shown as the areas Various modes ot carrying out the invention are con.- templated as being within the scope of the following claim Par ul ly p int g and is in t y claimi the subjectmatter which is regarded as the invention.

r, aim: A

ethod of making structural steel 'to be fabricated into an article subjected in service to an alkaline, medium, comprising adding to the ingotmold as the steel is, poured into said mold; iron oxide equivalent t0 at. least. 1.75 pounds of mill scale per ton of steel and at a rate deter; mined by the effervescence of the steel .to obtain a sub.- stantially uniformdistribution of thepxide-throughout the, ingot, said steel as poured having 'a carbon content in the range of 0; 15 ,to'.0.30% and said iron. oxide being a in theformoffine, particles added substantially continuin the etched sample, existfin the plateflandthese oxygeh i rich bands are disposed generallyparallel tothe plate sin; face, thereby afiording the] most f desiralple [resistance to corrosiveattack from alkaline media. I Hm The-banded condition of the oxidesgas, evidenced by the ato'rementioned etching p centuated" by'thermal treatment; By he'atingj the steel to Ia temperature abQVeIISOK F. and below ithcilower critical fternperaturefof the steel and maintaining the "ocedure, cari befurthe'r, Zac- 5 vouslyiduring the entire, pouring ,operationcapping the a i keieiefitfes' citainn ne aims lli fll 1 j U IrEDsTA sPATE f 

